The two primary methods of providing straight or flattened steel strip or sheet are roller leveling and stretcher leveling. Roller leveling is typically performed in a rolling machine consisting of two sets of rolls. A top and bottom set of several small diameter horizontal rolls each are mounted in a housing so that the associated top and bottom rolls are offset from each other. A steel sheet or strip passing through the leveler is flexed up and down alternately between the offset rolls such that the amount of flexing decreases as the sheet travels toward the exit end of the roller leveler. The rolls nearest the exit end are designed to perform the basic straightening operation. The advantage of roller leveling is that long lengths of sheets or strip may be leveled or leveled or flattened with minimum surface disfigurement. However, roller leveling does not impart the same degree of flatness to the sheet as a pair of opposing jaws actuated by hydraulic or pneumatic means.
Typically, sheet or strip is elongated between one and three percent so that the elastic limit of the steel is exceeded to produce permanent elongation. There are numerous types of stretcher leveler devices including those which can handle large coils of rolled strip. However, in all stretcher levelers the jaws of the device include gripping means to securely grip the opposing ends of the sheet which is to be stretched. These gripper means typically comprise a flat elongated engagement member having a length slightly greater than the width of the sheet or strip to be stretched. The surface of the engagement member which is adapted to engage or grip the surface of the sheet or strip to hold it against movement during elongation is very rough, normally grooved, knurled or serrated. Consequently, in virtually all such stretcher leveler devices the gripper means bite into the metal and disfigure the surface of the sheet. Traditionally, the disfigured portion of the sheet or strip is marked and subsequently cut off as scrap. For example, in a coil 2,125 feet in length, approximately 162" are lost in scrap.
The disfigurement of the metal results in substantial economic loss because that metal is normally discarded as waste. Moreover, when coils of rolled strips are stretched in sequential stretching, the gripper disfigurement marks must be indicated and cut from the coil. Thus, the maximum length of strip or sheet which could be leveled is the distance between the grippers.
U.S. Pat. No. 4,982,593 discloses a stretcher leveler apparatus having an element for gripping metal which does not disfigure the metal. The element has a single gripping surface comprised of high density cast polyurethane, which can grip the metal without slipping. U.S. Pat. No. 5,077,887 discloses a method of making a stretcher leveler gripping element wherein polyurethane is cast in situ onto a steel surface, to form a gripping pad which is permanently bonded to the support surface.
The present invention provides an improved gripper element which can be quickly replaced and withstand greater forces than heretofore known.